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PH-EP-2015-198

(Submitted on 04 Aug 2015)

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Abstract

Using a dataset corresponding to an integrated luminosity of 3.0 fb$^{-1}$
collected in $pp$ collisions at centre-of-mass energies of 7 and 8 TeV, the
$B_s^0 \to \phi \phi$ branching fraction is measured to be \[ \mathcal{B}(B_s^0
\to \phi \phi) = ( 1.84 \pm 0.05 (\text{stat}) \pm 0.07 (\text{syst}) \pm 0.11
(f_s/f_d) \pm 0.12 (\text{norm}) ) \times 10^{-5}, \] where $f_s/f_d$
represents the ratio of the $B_s^0$ to $B^0$ production cross-sections, and the
$B^0 \to \phi K^*(892)^0$ decay mode is used for normalization. This is the
most precise measurement of this branching fraction to date, representing a
factor five reduction in the statistical uncertainty compared with the previous
best measurement. A search for the decay $B^0 \to \phi \phi$ is also made. No
signal is observed, and an upper limit on the branching fraction is set as \[
\mathcal{B}(B^0 \to \phi \phi) < 2.8 \times 10^{-8} \] at 90% confidence level.
This is a factor of seven improvement compared to the previous best limit.

Figures and captions

The $K^+K^-K^+K^-$ invariant mass distribution.
The total fitted function as described in the text is shown by the (red) solid line,
the $ B ^0_ s \rightarrow \phi\phi$ component by the (blue) long-dashed line,
and the combinatorial background as the (purple) dotted line.

The $K^+K^-K^+K^-$ invariant mass with the tight BDT selection applied.
A fit to the total PDF as described in the text is shown as a (red) solid line,
$ B ^0_ s \rightarrow \phi\phi$ as a (blue) long-dashed line,
$ B ^0 \rightarrow \phi\phi$ as a (blue) short-dashed line,
and the combinatorial background as a (purple) dotted line.

Results of the $\text{CL}_\text{s}$ scan as a function of the $ B ^0 \rightarrow \phi\phi$ branching fraction ($BF$).
The observed $\text{CL}_\text{s}$ distribution is given by the (black) points and solid line,
while the expected distribution is given by the (black) dashed line.
The dark (green) and light (yellow) bands mark the $1\sigma$ and $2\sigma$ confidence regions on the expected $\text{CL}_\text{s}$.
The upper limits at 90$\,\%$ and 95$\,\%$ confidence level are where the observed $\text{CL}_\text{s}$ line intercepts the (red) solid and dashed horizontal lines, respectively.